Radio-wave Effect on Singlet Fission in Polycrystalline Tetracene near Zero Magnetic Field

A triplet–triplet pair is a key intermediate in singlet fission (SF), which has the potential to overcome the theoretical limit of solar cell efficiency. Here, we report a new spectroscopy to directly detect a short-lived triplet–triplet pair via the effects of radio-wave (RF) irradiation near zero magnetic field at room temperature. The fluorescence of polycrystalline powder of tetracene is reduced by RF irradiation at zero field, which is caused by a quasi-static RF field effect on spin mixing and electron-spin resonance among zero-field-splitting sublevels of the triplet–triplet pair. The curve for the quasi-static RF field effect can be reproduced numerically from that for the observed magnetophotoluminescence (MPL) effect. The simultaneous simulation of the RF and MPL effects using the density matrix formalism estimates rate constants of 1.2 × 108 and 6.0 × 108 s–1 for the fusion and dissociation, respectively, of the triplet–triplet pair.